A. Technical Field
The present invention relates to: a production process for a hydroxyalkyl (meth)acrylate, which comprises the step of carrying out a reaction between (meth)acrylic acid and an alkylene oxide in the presence of a catalyst; and a high-quality hydroxyalkyl (meth)acrylate which is obtained by this process.
B. Background Art
As to production processes for a hydroxyalkyl (meth)acrylate involving the step of carrying out a batch reaction between (meth)acrylic acid and an alkylene oxide in the presence of a catalyst, the following processes have hitherto been common: a process that involves the step of charging a reactor with (meth)acrylic acid and an alkylene oxide (which are raw materials) along with a catalyst in a lump to thus carry out their reaction; and a process that involves the steps of charging a reactor with (meth)acrylic acid (which is a raw material) and a catalyst and thereafter adding an alkylene oxide thereto gradually (continuously or intermittently) at a constant or variable supplying rate to thus carry out their reaction. The reasons therefor are such that: the (meth)acrylic acid has higher catalyst-dissolving ability than the alkylene oxide; and, if the alkylene oxide is initially charged alone into the reactor and then heated, there is involved a danger of explosion, and therefore special facilities for coping with this danger are needed. Even in the former process that involves the step of charging the reactor with the (meth)acrylic acid and the alkylene oxide along with the catalyst in a lump to thus carry out their reaction, strictly the fact is that the (meth)acrylic acid is charged earlier than the alkylene oxide.
However, both in the above processes, there is a problem such that an alkylene glycol di(meth)acrylate (which is a diester and may hereinafter be referred to simply as “diester”) and a dialkylene glycol mono(meth)acrylate (which is an alkylene oxide's diaddition product and may hereinafter be referred to simply as “diaddition product”) tend to be by-produced as impurities.
Of these by-products, the diaddition product can be removed by distillation, but the diester is difficult to remove even by purification such as distillation, therefore it is important to suppress the diester content at the end of the reaction. In the reaction between the (meth)acrylic acid and the alkylene oxide, the catalyst runs its substitution reaction with the (meth)acrylic acid to form a salt (e.g. chromium (meth)acrylate) to thereby activate the (meth)acrylic acid to thus promote its reaction with the alkylene oxide. However, it is conceived that: the (meth)acrylic acid concentration in the system decreases with the progress of the reaction, whereby the catalyst runs its substitution reaction not with the (meth)acrylic acid but with the formed hydroxyalkyl (meth)acrylate to cause a disproportionation reaction to by-produce the diester. Therefore, when an attempt is made to suppress the side production of the diester in the aforementioned prior processes, there has been adopted a method in which the reaction is early ended in a stage when a lot of (meth)acrylic acid remains unreacted. However, similarly to the diester, the residual acid component is difficult to remove by distillation. Therefore, as to the above method, though the diester content can be decreased, it is inevitable that the acid component content increases with this decrease of the diester content. Incidentally, herein, the acid component means all acids that are contained in the hydroxyalkyl (meth)acrylate and can be measured by neutralization titration.
Thus, as to prior arts, it is difficult to decrease the diester and the acid component at the same time, and it is the present state that, even if only either one of them comes to be contained in a large quantity, the resultant product is so low in purity as to be bad in quality.